1. Field of the Invention:
The present invention relates to a method for controlling the thickness of a sheet material and for monitoring a correspondence relationship between the thickness distribution across a sheet material and means for adjusting the thickness of the sheet material. More particularly the present invention relates to a thickness control method which can provide a sheet material at a target thickness and thickness distribution with high accuracy and can compensate for any variation in a sheet forming process, by obtaining an accurate correspondence relationship between a plurality of adjusting means disposed on a die and the thickness distribution of a formed sheet material, and to a monitoring method wherein feedback information useful for a sheet forming process can be obtained.
2. Description of the Prior Arts:
Various processes for producing a sheet material are known which include methods of controlling the thickness of, and the thickness distribution across, the sheet. In such processes a molten material, for example, a molten resin, is continuously delivered from a die in the form of a web and subjected to a sheet forming process, for example, a stretching process. The thickness of, and the thickness distribution across (i.e. in the width direction), the sheet material after formation thereof by the sheet forming process is measured by a thickness gauge, and a plurality of adjusting means arranged on the die in the width direction of the web of molten material are controlled so that the thickness distribution of the sheet material to be measured can approach a pre-determined target distribution.
For instance, such thickness control methods are disclosed in JP-A Nos. 52-36154, 52-36165, 56-120318, 56-133135, 58-78726, 60-132727 and 60-225730, mainly in processes for producing resin films. All these methods are based on the premise that a correspondence relationship between the position of the adjusting means on the die and the position in the width direction of the formed sheet material at which position the thickness of the sheet is varied by the adjustment of the adjusting means is fully determined prior to commencement of the sheet forming process.
However, in practice, in these conventional methods the above correspondence relationship can not be obtained with a high accuracy because, in such conventional methods, there are certain limits to the extent to which the accuracy of control of the thickness of the sheet material after formation by the sheet forming process can be increased.
For instance, a conventional sheet forming system for producing a biaxially oriented plastics film as a sheet material is schematically shown in FIG. 13. A web of molten resin 2 delivered from a die 1 is stretched biaxially by an apparatus 3 for longitudinally stretching the material and an apparatus 4 for transversely stretching the material so that a biaxially oriented plastics film 5 having a certain thickness is formed after passing through the system. The thickness distribution of the film 5 is measured, for example, by a scanning type thickness gauge 6. In a case where the width of the film 5 is 5-10 m, measuring positions of the thickness gauge 6 are provided, for example, at 300 points in the width direction of the film and a plurality of adjusting means disposed on the die 1, for example, adjusting bolts 7 for adjusting a gap provided by a slit of the die, are provided, there being for example, 100 in number.
In order to determine a correspondence relationship between the respective positions of each of the many such adjusting bolts 7 and the respective positions of the film 5 at which the thickness of material is varied by the adjustment of the respective bolts, that is, the influence of the adjustment of the bolts on the thickness of the film, it is usually necessary to carry out the following "forced" test. Namely, an appropriately pre-selected adjusting bolt 7 is greatly adjusted forcedly, in other words deliberately, using the system shown in FIG. 14, for example, by 20% of an acceptable adjusting amount. The position of the film 5 at which the thickness of the film greatly varies is determined and thus the correspondence relationship between the position of the adjusting bolt and the position of the film can be obtained.
In another, even less accurate, method of determining this correspondence relationship, the positions of the adjusting bolts 7 and the positions of the film 5 are merely allocated in the width direction of the film under the premise that the central position of the film in the width direction corresponds to the position of a center bolt of a plurality of bolts and/or under the premise that the positions of both edge portions of the film in the width direction correspond to the respective positions of both end bolts of the plurality of bolts. Each of these methods of mere allocation carries with it the assumption that the pitch (i.e. width) of each portion of the film the thickness of which is adjusted by a given respective bolt is the same for each portion.
However, this is not usually the case, because the thickness of a desired film usually varies across its width, being thicker at its edges. Accordingly the respective pitches at edge portions are narrower.
This enables the measured thickness to be used to give some idea of which bolt influences the thickness of a particular portion of the film without the nesessity for the above methods of allocation.
Alternatively, this variation in thickness across the film may be used as a refinement enabling a more accurate allocation of the positions of the bolts by the abovementioned methods.
However, even if the correspondence relationship between the respective positions of adjusting bolts 7 and the positions of the film 5 is determined by the abovementioned, slightly more accurate method using the system shown in FIG. 14, it is necessary to adjust a particular bolt 7 to a fairly great extent and to forcedly generate a great thickness variation at a particular position on the film so that the thickness variation can be clearly recognized as in thickness profile 8. A film having such a great nonuniformity of thickness distribution cannot provide a uniform product, and hence the above test, in effect, requires a break-down of the process. Accordingly, a great loss of film is generated during the test, and it takes a considerable period of time to readjust the forcedly operated bolts to their original respective settings and for the thickness and thickness distribution of the film to settle within ranges for which the film can be obtained as a uniform, and therefore useful, product, thereby decreasing the productivity greatly.
Moreover, in the method wherein the correspondence relationship between the positions of adjusting bolts 7 and the positions of the film 5 in the width direction is determined by allocation according to a uniform pitch or mass ratio, or in the method wherein, after the deliberate adjustment test providing a considerable change in thickness as shown in FIG. 14 is carried out in accordance with a procedure in which a plurality of adjusting bolts 7 are appropriately pre-selected, and the remaining bolts between the test bolts are allocated to the positions of the film with a uniform pitch or according to the mass ratio, it is almost impossible to obtain an accurate correspondence relationship between the positions of the bolts and the positions of the film in the width direction, this being because there are stretching processes, particularly the transverse-stretching process carried out by stretching apparatus 4, and uniform stretching is not always performed by this process.
Furthermore, since the conditions of respective film forming processes performed by apparatus 3 and 4 cannot always be constant conditions in themselves or in relation to the film, the treatment conditions and/or stretching conditions may vary. As a result of the variations, the correspondence relationship between the positions of the adjusting bolts 7 and the positions of the film 5 in the width direction varies as shown by arrows 9 in FIG. 13, so that it becomes much more difficult to obtain an accurate relationship.
Thus, since an accurate correspondence relationship cannot be obtained in practice by conventional methods, the bolts 7 are often inadequately adjusted in response to the feedback signals of thickness distribution of the film 5 from the thickness gauge 6, even if accurate measurement can be performed by the thickness gauge. As a result, there is inevitably a certain limit to the extent to which the accuracy of the thickness control of the film 5 can be improved.
Further, there is a problem associated with the degree of adjustment to be made to the adjusting bolts 7. Even if the variation in thickness of the film 5 at a particular position in the width direction where the thickness is to be altered by the deliberate adjustment of the bolt 7 (for example, a 20% adjustment) can be measured, the entire relationship between the degree of adjustment of the bolt and the remitting thickness variation of the film cannot be precisely obtained. Thus, the degree of adjustment of the bolt 7 to be made in the actual production of a useful film 5 is very small, for example, 0.1-0.5%, and the actual amount of variation of the thickness of the film due to such a small adjustment of the bolt does not always become equal to a value calculated proportionally from the above relationship derived from the above test in which the bolts are actively adjusted. The main reason for this is that there are forming processes provided by the stretching apparatus 3 and 4 between the die 1 and the thickness gauge 6. Thus the relationship between the degree of adjustment of the adjusting bolt 7 and the thickness variation of the film 5 is also a complicated correspondence relationship. Still more, in a case where the conditions of the processes carried out by stretching apparatus 3 and 4 fluctuate, it is impossible to determine the correspondence relationship constantly and accurately.
Furthermore, in addition to the abovementioned problem, we have also recognized the following problem associated with the correspondence relationship between the positions of the adjusting bolts 7 and the positions of the film 5 in the width direction.
According to a fairly accurate test which we have carried out, when the adjusting bolt 7 on the die 1 is adjusted by a small unit amount (for example, 0.1-0.5% of an acceptable adjusting amount), this results in a pattern of the variation in thickness of the film 5 at the position where the thickness is varied (this position can be pre-determined by a test such as the abovementioned forced test in which the bolts 7 are actively adjusted to provide the result shown in FIG. 14), for example, as shown in FIG. 15. By a unit amount of adjustment of the bolt 7, the thickness of the film 5 around the position corresponding to the position of the bolt varies as shown by thickness profile 10. At the same time, the thicknesses of both respective portions at either side of the above position of the film 5 in the width direction vary as shown by thickness profiles 11a and 11b, in a thickness direction opposite to the thickness direction of the portion of the thickness profile 10. This pattern results from the fact that when a portion of the molten resin in the die 1 is locally thickened, the molten resin in both side portions of the above central portion in the width direction of the film is drawn toward the central portion due to the flow of the molten resin at the central portion, and the influence of this variation at the die is revealed in the thickness profile of the film 5 after formation. Since the central portion having the thickness profile 10 and the two side portions having the thickness profiles 11a and 11b of the film 5 vary by the unit amount of adjustment of the adjusting bolt 7 in respective directions opposite to each other, if the correspondence relationship between the positions of the adjusting bolts and the positions of the film is not determined accurately as aforementioned, the portion to be adjusted so as to increase the thickness of the film may often be thinned or the portion to be adjusted so as to decrease the thickness may often be thickened. Thus, for this reason also, there exists an unsatisfactory limit to the accuracy of the thickness control of the sheet material which can be achieved by conventional methods.